The present invention relates to a range finder of a type employing a semiconductor optical position detecting element used, for instance, in a camera.
Automatic focusing devices using an infrared active system are well known in the art. In the so-called infrared active system, a thin infrared beam reflected from the object is detected to measure the distance from the object. This system is advantageous in that the distance can be accurately measured irrespective of the brightness or contrast of the ordinary light reflected from the object. Furthermore, it is unnecessary to scan a whole picture and to precisely focus the distance measuring optical system on the object.
The infrared active system extensively usually employs a semiconductor optical position detecting element (hereinafter referred to as a PSD when applicable). A conventional range finder using such a PSD is arranged as shown in FIG. 1. In FIG. 1, a light projecting lens 1 and a condenser lens 2 have a focal length f. A light beam from an infrared source 4 is applied through the light projecting lens 1 to an object 3. The light projecting lens 1 and the condenser lens 2 are set a base length D apart from each other, and the optical axis of the condenser lens 2 passes through the center of the PSD 5.
The condenser lens 2 gathers the light reflected from the object 3, thus forming a light spot at a point which is .DELTA.L from the center of the PSD 5 and offset from the infrared source 4. As a result, currents I.sub.1 and I.sub.2, which are inversely proportional to the resistances between the position of the light spot and the electrodes A and B, respectively, flow to the electrodes A and B, respectively, of the PSD 5.
FIG. 2 shows a position detecting circuit 6. The circuit 6 includes a differential amplifier 7 for obtaining a differential current I.sub.D (=I.sub.1 -I.sub.2) and a summing amplifier 8 for obtaining a sum current I.sub.A (=I.sub.1 +I.sub.2). The circuit 6 further includes an analog divider 9 for obtaining a division current I.sub.d (=I.sub.D /I.sub.A). By dividing the current ID by the current IA, an accurate distance signal can be obtained at all times. The distance signal is independent of the intensity of the light reflected from the object.
In the position detecting circuit of FIG. 2, the output signal is obtained by processing currents; however, it may be obtained by processing voltages or by processing both currents and voltages in combination.
In the above-described conventional range finder, the infrared beam is very thin, the beam diameter at the object being not more than 10 cm. Therefore, if the main object to be photographed is located at the side of the picture, the infrared beam may not be applied to the object, and accordingly the distance thereto is measured with error.
In order to prevent this difficulty, a focus locking operation is generally carried out. That is, the camera is moved so that the object is at the center of the picture, and the shutter button is depressed halfway to apply the light beam to the object, thereby to measure the distance. Under this condition, the camera is moved back to the original position, and the shutter button completely depressed
This operation is disadvantageous in that it is considerably troublesome In addition, an unskilled photographer often does not know how to conduct the focus locking operation and is thus liable to take many pictures out of focus. Even if the photographer is sufficiently skilled to use the focus locking operation, because it is time consuming, many picture taking opportunities are missed.
In most cases where pictures are taken out of focus, the composition is of two persons standing side by side. In such a composition, the light beam passes through the space between the two persons so that the background is focused. In the case of three persons, the central person is usually positioned at the center of the picture, and therefore the focusing light beam is correctly applied, and thus such pictures are taken in focus.